Sensor for detecting electrically conductive and/or polarizable particles, sensor system, method for operating a sensor, method for producing a sensor of this type and use of a sensor of this type

ABSTRACT

A sensor for detecting electrically conductive and/or polarizable particles, in particular for detecting soot particles, includes a substrate and at least two electrode layers, a first electrode layer and at least one second electrode layer. Which is arranged between the substrate and the first electrode layer. At least one insulation layer is formed between the first electrode layer and the at least one second electrode layer and at least one opening is formed in both the first electrode layer and the at least one insulation layer. At least some sections of the opening in the first electrode layer and of the opening in the insulation layer are arranged one above the other, such that at least one passage is formed to the second electrode layer.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The invention is directed to a sensor for detecting electricallyconductive and/or polarizable particles, in particular for detectingsoot particles. The invention is also directed to a sensor system, to amethod for operating a sensor, to a method for producing a sensor fordetecting electrically conductive and/or polarizable particles and to ause of a sensor of this type.

2. Discussion of the Related Art

The prior art discloses sensors comprising a sensor carrier, withelectrodes and heating structures being arranged on this sensor carrierin a planar arrangement. In a detecting mode of operation, polarizableand/or electrically conductive particles are deposited on this planararrangement. The deposited particles bring about a reduction in theresistance between the electrodes, this drop in the resistance beingused as a measure of the mass of deposited particles. When a predefinedthreshold value with respect to the resistance is reached, the sensorarrangement is heated by the heating structures, so that the depositedparticles are burned and, after the cleaning process, the sensor can beused for a further detection cycle.

DE 10 2005 029 219 A1 gives a description of a sensor for detectingparticles in an exhaust-gas flow of internal combustion engines, theelectrode, heater and temperature-sensor structures having been appliedto a sensor carrier in a planar arrangement. One disadvantage of thissensor arrangement is that the electrodes to be bridged have a necessaryminimum length in order to be able to arrive at an acceptablesensitivity range when measuring conductive or polarizable particles,such as for example soot. However, a certain size of the sensorcomponent is necessary for this, in order to be able to arrange theminimum length for the electrodes to be bridged. This is accompanied bycorresponding cost disadvantages in the production of these sensorcomponents.

The invention is based on the object of providing a further-developedsensor for detecting electrically conductive and/or polarizableparticles, in particular for detecting soot particles, the sensor beingminimized with regard to its size, so that the aforementioneddisadvantages can be overcome.

The object of the present invention is also to provide a sensor system,a method for operating a sensor and a method for producing a sensor ofthis type.

SUMMARY OF THE INVENTION

This object is achieved according to the invention by a sensor fordetecting electrically conductive and/or polarizable particles, inparticular for detecting soot particles.

The invention is based on the idea of providing a sensor for detectingelectrically conductive and/or polarizable particles, in particular fordetecting soot particles, comprising a substrate and at least twoelectrode layers, a first electrode layer and at least a secondelectrode layer, which is arranged between the substrate and the firstelectrode layer, being arranged, at least one insulation layer beingformed between the first electrode layer and the at least a secondelectrode layer and at least one opening being respectively formed inthe first electrode layer and in the at least one insulation layer, theopening in the first electrode layer and the opening in the insulationlayer being arranged at least in certain portions one over the other insuch a way that at least one passage to the second electrode layer isformed.

A sensor is preferably provided, comprising a substrate, a firstelectrode layer, a second electrode layer, which is arranged between thesubstrate and the first electrode layer, a first insulation layer beingformed between the first electrode layer and the second electrode layer,at least a third electrode layer being formed between the firstinsulation layer and the first electrode layer, and at least a secondinsulation layer being formed between the at least third electrode layerand the first electrode layer, at least one opening being respectivelyformed in the first electrode layer, in the at least second insulationlayer, in the at least third electrode layer and in the first insulationlayer, the opening in the first electrode layer, the opening in the atleast second insulation layer, the opening in the at least thirdelectrode layer and the opening in the insulation layer being arrangedat least in certain portions one over the other in such a way that atleast one passage to the second electrode layer is formed.

In other words, a sensor is made available, a first and a secondelectrode layer being arranged horizontally one over the other and afirst insulation layer, optionally at least a third electrode layer andoptionally at least a second insulation layer being formed between thesetwo electrode layers. In order to form a passage to the second electrodelayer, so that particles to be detected, in particular soot particles,can reach the second electrode layer with the aid of the passage, boththe first and third electrode layers and the first and second insulationlayers respectively have at least one opening, the opening in the firstand third electrode layers and the opening in the first and secondinsulation layers being arranged at least in certain portions one overthe other, so that the passage is formed or can be formed.

Particles can accordingly reach the second electrode layer by way of atleast one passage only from one side of the sensor, to be specific fromthe side of the sensor that is made to be the closest to the firstelectrode layer. The electrically conductive and/or polarizableparticles accordingly lie on a portion of the second electrode layer.

The sensor according to the invention may for example comprise at leastthree electrode layers and at least two insulation layers, an insulationlayer preferably always being formed between two electrode layers.

An insulation layer may also consist of two or more sublayers, which maybe arranged next to one another and/or one over the other. Two or moresublayers of an insulation layer may consist of different materialsand/or comprise different materials.

An electrode layer may also consist of two or more sublayers, which maybe arranged next to one another and/or one over the other. Two or moresublayers of an electrode layer may consist of different materialsand/or comprise different materials.

It is possible that the sensor comprises more than three electrodelayers and more than two insulation layers, also in this situation aninsulation layer preferably always being formed between two electrodelayers. From now on, the expression “at least third electrode layer”should be understood as meaning that a fourth and/or fifth and/or sixthand/or seventh and/or eighth and/or ninth and/or tenth electrode layermay also be intended instead of the stated third electrode layer.

From now on, the expression “at least second insulation layer” should beunderstood as meaning that a third and/or fourth and/or fifth and/orsixth and/or seventh and/or eighth and/or ninth insulation layer mayalso be intended instead of the stated second insulation layer.

The sensor according to the invention may in other words comprise alaminate which comprises at least three electrode layers and at leasttwo insulation layers. The electrode layer closest to the substrate isreferred to as the second electrode layer, the electrode layer at themaximum distance from the substrate is referred to as the firstelectrode layer. Between the first electrode layer and the secondelectrode layer there is for example at least a third electrode layer,at least one insulation layer being respectively formed between twoelectrode layers.

The electrode layers are arranged one over the other, in particular inlayers one over the other, the electrode layers being respectively keptat a distance from one another by means of the insulation layers. Inother words, the electrode layers do not lie in one plane.

Preferably, the opening in the first electrode layer is formed at adistance from the peripheral region of the first electrode layer, theopening in the optionally at least second insulation layer is formed ata distance from the peripheral region of the second insulation layer,the opening in the optionally at least third electrode layer is formedat a distance from the peripheral region of the third electrode layerand the opening in the first insulation layer is formed at a distancefrom the peripheral region of the first insulation layer. The openingsare accordingly preferably not formed in a peripheral position, or notformed at the side peripheries of the layers concerned.

The first electrode layer and the optionally third electrode layer areinsulated from one another by the second insulation layer located inbetween. The optionally third electrode layer and the second electrodelayer are insulated from one another by the first insulation layerlocated in between. Such a structure allows a very sensitive sensor of asmaller overall size in comparison with sensors of the prior art to beformed.

The second electrode layer, formed for example with a flat extent, isindirectly or directly connected to the substrate. An indirectconnection of the second electrode layer to the substrate may take placefor example by means of a bonding agent, in particular a bonding agentlayer. The bonding agent may also be formed in an insular manner betweenthe substrate and the second electrode layer. For example, a drop-likeformation of the bonding agent/the bonding agent layer is possible. Abonding agent layer may be formed between the second electrode layer andthe substrate.

The bonding agent, in particular the bonding agent layer, may forexample consist of an aluminum oxide (Al₂O₃) or a silicon dioxide (SiO₂)or a ceramic or a glass or any desired combinations thereof. The bondingagent layer is preferably formed very thin, and consequently only has asmall thickness.

The first insulation layer and/or the at least second insulation layermay have a thickness of 0.1 to 50 μm, in particular of 1.0 μm to 40 μm,in particular of 5.0 μm to 30 μm, in particular of 7.5 μm to 20 μm, inparticular of 8 μm to 12 μm. With the aid of the thickness of theinsulation layer(s), the distance of one electrode layer from anotherelectrode layer is set. The sensitivity of the sensor can be increasedby reducing the distance between the, for example flat-extending,electrode layers, located one over the other. The smaller the thicknessof the insulation layer is formed, the more sensitive the sensor ismade.

It is also possible that the thickness(es) of the electrode layersand/or the thickness(es) of the insulation layer(s) of a substrate vary.

The insulation layer(s) may be formed from aluminum oxide (Al₂O₃) orsilicon dioxide (SiO₂) or magnesium oxide (MgO) or silicon nitride(Si₂N₄) or glass or ceramic or any desired combinations thereof.

Preferably, the first insulation layer laterally encloses the secondelectrode layer. In other words, the first insulation layer can coverthe side faces of the second electrode layer in such a way that thesecond electrode layer is laterally insulated. For example, the at leastsecond insulation layer laterally encloses the at least third electrodelayer. In other words, the second insulation layer can cover the sidefaces of the third electrode layer in such a way that the thirdelectrode layer is laterally insulated.

The first electrode layer and/or the second electrode layer and/or theoptionally at least third electrode layer is formed from a conductivematerial, in particular from metal or an alloy, in particular from ahigh-temperature-resistant metal or a high-temperature-resistant alloy,particularly preferably from a platinum metal or from an alloy of aplatinum metal. The elements of the platinum metals are palladium (Pd),platinum (Pt), rhodium (Rh), osmium (Os), iridium (Ir) and ruthenium(Rh). Nonprecious metals such as nickel (Ni) or nonprecious metal alloyssuch as nickel/chromium or nickel/iron may also be used.

It is possible that at least one electrode layer is formed from aconductive ceramic or a mixture of metal and ceramic. For example, atleast one electrode layer may be formed from a mixture of platinumgrains (Pt) and aluminum oxide grains (Al₂O₃). It is also possible thatat least one electrode layer comprises silicon carbide (SiC) or isformed from silicon carbide (SiC). The stated materials and metals oralloys of these metals are particularly high-temperature-resistant andare accordingly suitable for the forming of a sensor element that can beused for detecting soot particles in an exhaust-gas flow of internalcombustion engines.

In a further embodiment of the invention, the second electrode layer isformed from a conductive material, in particular from a metal or analloy, that has a higher etching resistance than the conductivematerial, in particular the metal or the alloy, of the first electrodelayer. This has the advantage that the second electrode layer can beformed in a production process as a layer stopping the etching process.In other words, a second electrode layer formed in this way candetermine the depth to be etched of a passage that is for example to beintroduced into the sensor structure.

On the side of the first electrode layer that is facing away from thefirst insulation layer there may be formed at least one covering layer,which is formed in particular from ceramic and/or glass and/or metaloxide. In other words, the covering layer is formed on a side of thefirst electrode layer that is opposite from the first insulation layer.The covering layer may serve as a diffusion barrier and additionallyreduces an evaporation of the first electrode layer at hightemperatures, which in an exhaust-gas flow for example may be up to 850°C.

The at least one covering layer may laterally enclose the firstelectrode layer. In a further embodiment of the invention, the coveringlayer may additionally laterally enclose the at least second insulationlayer. In a further embodiment of the invention, the covering layer mayadditionally laterally enclose the at least second insulation layer andthe at least third electrode layer.

It is possible that at least one covering layer does not completelycover the uppermost electrode layer, in particular the first electrodelayer. In other words it is possible that at least one covering layeronly covers certain portions of the uppermost electrode layer, inparticular the first electrode layer. If the uppermost electrode layeris formed as a heating layer, it is possible that only the portions ofthe heating loop/heating coil are covered by the at least one coveringlayer.

In a further embodiment of the invention, the at least one coveringlayer may additionally laterally enclose the at least second insulationlayer and the at least third electrode layer and the first insulationlayer. In other words, both the side faces of the first electrode layerand the side faces of the insulation layers and electrode layersarranged thereunder may be covered by at least one covering layer. It isalso conceivable that the covering layer additionally laterally enclosesthe second electrode layer. The lateral enclosing part or lateralenclosing region of the covering layer may accordingly reach from thefirst electrode layer to the second electrode layer. This brings about alateral insulation of the first electrode layer and/or of the insulationlayers and/or of the at least third electrode layer and/or of the secondelectrode layer.

On the side of the first electrode layer that is facing away from thefirst insulation layer or on the side of the covering layer that isfacing away from the first electrode layer there may be additionallyformed at least one porous filter layer. With the aid of a porous filterlayer of this type, large particle parts can be kept away from thearrangement of at least two, in particular at least three, electrodelayers arranged one over the other. The pore sizes of the filter layermay be for example >1 μm. Particularly preferably, the pore size isformed in a range from 20 μm to 30 μm. The porous filter layer may forexample be formed from a ceramic material. It is also conceivable thatthe porous filter layer is formed from an aluminum oxide foam. With theaid of the filter layer, which also covers the at least one passage tothe second electrode layer, the large particles, in particular sootparticles, that disturb the measurement can be kept away from the atleast one passage, so that such particles cannot cause a short circuit.

The at least one passage to the second electrode layer may for examplebe formed as a blind hole, a portion of the second electrode layer beingformed as the bottom of the blind hole and the blind hole extending atleast over the first insulation layer, over the optionally at leastthird electrode layer, over the optionally at least second insulationlayer and over the first electrode layer. If the sensor has a coveringlayer, the blind hole also extends over this covering layer. In otherwords, not only the first electrode layer but also the optionally atleast second insulation layer, the optionally at least third electrodelayer and the first insulation layer and the covering layer then have anopening, these openings being arranged one over the other in such a waythat they form a blind hole, the bottom of which is formed by a portionof the second electrode layer. The bottom of the blind hole may forexample be formed on the upper side of the second electrode layer thatis facing the first insulation layer. It is also conceivable that thesecond electrode layer has a depression that forms the bottom of theblind hole.

The opening cross section of the blind hole is formed by the peripheralportions of the first electrode layer, of the at least second insulationlayer, of the at least third electrode layer and of the first insulationlayer and, if there is one, of the covering layer that bound theopenings. The opening cross section of the at least one blind hole maybe round or square or rectangular or lenticular or honeycomb-shaped orpolygonal or triangular or hexagonal. Other types of design, inparticular free forms, are also conceivable.

For example, it is possible that the blind hole has a square crosssection with a surface area of 3×3 μm² to 150×150 μm², in particular of10×10 μm² to 100×100 μm², in particular of 15×15 μm² to 50×50 μm², inparticular of 20×20 μm².

In a development of the invention, the sensor may have a multiplicity ofpassages, in particular blind holes, these blind holes being formed asalready described. It is also conceivable that at least two passages, inparticular at least two blind holes, have different cross sections, inparticular different sizes of cross section, so that a sensor array witha number of zones can be formed, in which a number of measuring cellswith blind-hole cross sections of different sizes can be used. Paralleldetection of electrically conductive and/or polarizable particles, inparticular of soot particles, allows additional items of informationconcerning the size of the particles or the size distribution of theparticles to be obtained.

In a further embodiment of the invention, the openings in the firstinsulation layer, in the optionally at least third electrode layer, inthe optionally at least second insulation layer, and in the firstelectrode layer may be respectively formed in a linear form orrespectively formed in a meandering manner or respectively formed in agrid form or respectively formed in a spiral form. In other words, anopening in the first insulation layer, an opening in the optionally atleast third electrode layer, an opening in the optionally at leastsecond insulation layer, and an opening in the first electrode layer arerespectively formed in a linear form or respectively formed in ameandering manner or respectively formed in a spiral form orrespectively formed in a grid form. The openings in the individuallayers are preferably formed similarly, so that a passage can be formed.The openings do not necessarily have to have exactly coinciding crosssections or exactly coinciding sizes of cross section. It is possiblethat, beginning from the second electrode layer, the cross sections ofthe openings respectively become greater in the direction of the firstelectrode layer. The basic forms of the openings are preferably formedsimilarly, so that all of the openings are formed either in a linearform or in a meandering manner or in a spiral form or in a grid form.

In a further embodiment of the invention it is possible that the sensorhas a number of passages that are formed in a linear form and/or ameandering manner and/or a spiral form and/or a grid form.

If the second electrode layer has the form of a meander or the form of aloop, the at least one passage of the sensor is formed in such a waythat the passage does not end in a gap or an opening in the form of themeander or the form of the loop. The at least one passage of the sensoris formed in such a way that a portion of the second electrode layerforms the bottom of the passage.

It is also possible that the at least one passage is formed as anelongate depression, a portion of the second electrode layer beingformed as the bottom of the elongate depression and the elongatedepression extending at least over the first insulation layer, over theoptionally at least third electrode layer, over the optionally at leastsecond insulation layer, and over the first electrode layer and overa/the optionally formed covering layer.

The elongate depression may also be referred to as a trench and/orgroove and/or channel.

In a further embodiment of the invention it is possible that the sensorcomprises both at least one passage in the form of a blind hole, whichis formed as round and/or square and/or rectangular and/or lenticularand/or honeycomb-shaped and/or polygonal and/or triangular and/orhexagonal, and at least one passage in the form of an elongatedepression, which is formed in a linear form and/or a meandering mannerand/or in a spiral form and/or in a grid form.

In a further embodiment of the invention, the first electrode layer, theoptionally at least second insulation layer, the optionally at leastthird electrode layer and the first insulation layer are respectivelyformed as porous, the at least one opening in the first electrode layer,the at least one opening in the optionally at least second insulationlayer, the at least one opening in the optionally at least thirdelectrode layer, and the at least one opening in the first insulationlayer respectively being formed by at least one pore, the pore in thefirst insulation layer, the pore in the at least third electrode layer,the pore in the at least second insulation layer and the pore in thefirst electrode layer being arranged at least in certain portions oneover the other in such a way that the at least one passage to the secondelectrode layer is formed. In other words, it is possible to dispensewith an active or subsequent structuring of the passages, the first andat least third electrode layer and the first and at least secondinsulation layer being formed as permeable to the medium to be measured.

This can be made possible for example by a porous or granular structureof the layers. Both the electrode layers and the insulation layers canbe produced by sintering together individual particles, with pores orvoids for the medium to be measured being formed while they are beingsintered together. The second electrode layer is preferably formed asnon-porous. Accordingly, at least one passage that allows access to thesecond electrode layer for a particle that is to be measured or detectedmust be formed, extending from the side of the first electrode layerthat is facing away from the first insulation layer to the side of thesecond electrode layer that is facing the insulation layer as a resultof the one-over-the-other arrangement of pores in the electrode layers,in particular the first and the optionally at least third electrodelayer, and in the insulation layers. If the sensor has a covering layer,this covering layer is also preferably to be formed as porous in such away that a pore in the covering layer, a pore in the first electrodelayer, a pore in the second insulation layer, a pore in the thirdelectrode layer and a pore in the first insulation layer form a passageto the second electrode layer.

The pore size distribution and their number in the first and optionallythird electrode layer and/or the first and optionally second insulationlayer and/or the covering layer(s) can be optimized with regard to themeasuring or detecting tasks to be carried out.

The first and/or third electrode layer and/or the first and/or secondinsulation layer and, if there is one, the at least one covering layermay have portions with different pore sizes in such a way that a sensorarray with a number of zones of different pore sizes is formed. Paralleldetection with portions of layers of different pore sizes allows a“fingerprint” of the medium that is to be analyzed or detected to bemeasured. Accordingly, further items of information concerning the sizeof the particles to be measured or the size distribution of theparticles to be measured can be obtained.

The first electrode layer, the second electrode layer and the optionallyat least third electrode layer respectively have an electricalcontacting area that are free from sensor layers arranged over therespective electrode layers and are or can in each case be connected toa terminal pad. The electrode layers are connected or can be connectedto terminal pads in such a way that they are insulated from one another.For each electrode layer there is formed at least one electricalcontacting area, which is exposed in the region of the terminal pads forthe electrical contacting. The electrical contacting area of the firstelectrode layer is free from a possible covering layer and free from apassive porous filter layer. In other words, above the electricalcontacting area of the first electrode layer there is neither a portionof the covering layer nor a portion of the filter layer.

The electrical contacting area of the second or at least third electrodelayer is free from insulation layers, free from electrode layers, andalso free from a possibly formed covering layer and free from a passiveporous filter layer.

In other words, on the electrical contacting area of the second or atleast third electrode layer there is neither a portion of an insulationlayer nor a portion of an electrode layer, nor a portion of the passiveporous filter layer.

In a further embodiment of the invention, the first electrode layerand/or the second electrode layer and/or the at least third electrodelayer has strip conductor loops in such a way that the first electrodelayer and/or the second electrode layer and/or the at least thirdelectrode layer is formed as a heating coil and/or as atemperature-sensitive layer and/or as a shielding electrode. The firstelectrode layer and/or the second electrode layer and/or the at leastthird electrode layer has at least one additional electrical contactingarea that is free from sensor layers arranged over the electrode layer,that is to say the first and/or the second and/or the at least thirdelectrode layer, and is connected or can be connected to an additionalterminal pad. In other words, the first electrode layer and/or thesecond electrode layer and/or the at least third electrode layer has twoelectrical contacting areas, both electrical contacting areas being freefrom sensor layers arranged over the electrode layer.

The formation of two electrical contacting areas on an electrode layeris necessary whenever this electrode layer is formed as a heating coiland/or temperature-sensitive layer and/or as a shielding electrode.Preferably, the second and/or the at least third electrode layer has atleast two electrical contacting areas. The second and/or the at leastthird electrode layer is preferably formed not only as a heating coilbut also as a temperature-sensitive layer and as a shielding electrode.By appropriate electrical contacting of the electrical contacting area,the electrode layer can either heat or act as a temperature-sensitivelayer or shielding electrode. Such a formation of the electrode areasallows compact sensors to be provided, since one electrode layer canassume a number of functions. Accordingly, no separate heating coillayers and/or temperature-sensitive layers and/or shielding electrodelayers are necessary.

During the heating of at least one electrode layer, measured particlesor particles located in a passage of the sensor may for example beburned away or burned off.

To sum up, it can be stated that a very accurately measuring sensor canbe made available as a result of the structure according to theinvention. The forming of a/a number of thin insulation layers allowsthe sensitivity of the sensor to be increased significantly.

Furthermore, the sensor according to the invention can be made muchsmaller than known sensors. The formation of the sensor in athree-dimensional space allows a number of electrode layers and/or anumber of insulation layers to be built as a smaller sensor.Furthermore, significantly more units can be formed on a substrate or awafer during the production of the sensor. This structure isconsequently accompanied by a considerable cost advantage in comparisonwith normally planar-constructed structures.

A further advantage of the sensor according to the invention is that thecross sections of the passages can be dimensioned in such a way thatspecific particles of specific sizes cannot enter the passages. It isalso possible that the cross sections of a number of passages can be ofdifferent sizes, so that only specific particles of correspondingparticle sizes are allowed access into individual passages.

The sensor according to the invention may be used for detectingparticles in gases. The sensor according to the invention may be usedfor detecting particles in liquids. The sensor according to theinvention may be used for detecting particles in gases and liquids orgas-liquid mixtures. When the sensor is used for detecting particles inliquids, it is not always possible however to burn off or burn away theparticles.

In the case of known sensors, the sensors are arranged in one plane andengage in one another. In the case of the present sensor, it is notnecessary for the electrode structures to engage in one another, sincethe individual electrode layers are formed at a distance from oneanother as a result of the formation of insulation layers between theelectrode layers. The electrode layers of the sensor according to theinvention are not connected to one another, but lie one over the other,separated by at least one insulation layer. There is a “non continuousloop” between at least a first electrode layer and at least a secondelectrode layer. The at least two electrode layers are not twistedtogether or entwined. At least two electrode layers can only beelectrically connected to one another by a soot particle located in atleast one passage.

With the aid of at least three formed electrode layers, it is possibleduring a measurement of particles for example to deduce the particlesize or to detect the particle size. If a particle bridges only twoelectrode layers arranged one over the other, the size of the particleis smaller than a particle that bridges more than two electrode layers.Different formations of the thickness of the insulation layers alsoallow the size of the particles to be deduced.

According to an independent aspect, the invention relates to a sensorsystem, comprising at least one sensor according to the invention and atleast one controller, in particular at least one control circuit, whichis formed in such a way that the sensor can be operated in a measuringmode and/or in a cleaning mode and/or in a monitoring mode.

The sensor according to the invention and/or the sensor system accordingto the invention may have at least one auxiliary electrode. Between anauxiliary electrode and an electrode layer and/or between an auxiliaryelectrode and a component of the sensor system, in particular the sensorhousing, there may be applied such an electrical potential that theparticles to be measured are electrically attracted or sucked in by thesensor and/or the sensor system. Preferably, such a voltage is appliedto the at least one auxiliary electrode and to at least one electrodelayer that particles, in particular soot particles, are “sucked into”the at least one passage.

The sensor according to the invention is preferably arranged in a sensorhousing. The sensor housing may for example have an elongate tube form.The sensor system according to the invention may accordingly alsocomprise a sensor housing.

Preferably, the sensor and/or the sensor in the sensor housing and/orthe sensor housing is formed and/or arranged in such a way that thesensor, in particular the uppermost layer of the sensor, or the layer ofthe sensor that is arranged furthest away from the substrate, isarranged obliquely in relation to the direction of flow of the fluid.The flow in this case does not impinge perpendicularly on the plane ofthe electrode layers. Preferably, the angle α between the normal to theplane of the first electrode layer and the direction of flow of theparticles is at least 1 degree, preferably at least 10 degrees,particularly preferably at least 30 degrees. Also preferred is anarrangement of the sensor in which the angle β between the direction offlow of the particles and the longitudinal axis of for example elongatedepressions lies between 20 and 90 degrees. In this embodiment, theparticles to be detected more easily enter the passages, in particularblind holes or elongate depressions, in the sensor, and thereby increasethe sensitivity.

The controller, in particular the control circuit, is preferably formedin such a way that the electrode layers of the sensor are interconnectedwith one another. Such voltages may be applied to the electrode layersor individual electrode layers that the sensor can be operated in ameasuring mode and/or in a cleaning mode and/or in a monitoring mode.

According to an independent aspect, the invention relates to a methodfor controlling a sensor according to the invention and/or a sensorsystem according to the invention.

The method according to the invention allows the sensor to be operatedaccording to choice in a measuring mode and/or in a cleaning mode and/orin a monitoring mode.

In the measuring mode, a change in the electrical resistance between theelectrode layers or between at least two electrode layers of the sensorand/or a change in the capacitances of the electrode layers can bemeasured.

With the aid of the method according to the invention, particles can bedetected or measured on the basis of a measured change in resistancebetween the electrode layers and/or by a measurement of the change inimpedance and/or by a measurement of the capacitance of the electrodelayer(s). Preferably, a change in resistance between the electrodelayers is measured.

In the measuring mode, an electrical resistance measurement, that is tosay a measurement on the resistive principle, may be carried out. Thisinvolves measuring the electrical resistance between two electrodelayers, the electrical resistance decreasing if a particle, inparticular a soot particle, bridges at least two electrode layers, whichact as electrical conductors.

It applies in principle in the measuring mode that, by applyingdifferent voltages to the electrode layers, different properties of theparticles to be measured, in particular soot particles, can be detected.For example, the particle size and/or the particle diameter and/or theelectrical charging and/or the polarizability of the particle can bedetermined.

If at least one electrode layer is also used or can be connected as aheating coil or heating layer, an electrical resistance measurement mayadditionally serve the purpose of determining the point in time of theactivation of the heating coil or heating layer. The activation of theheating coil or heating layer corresponds to a cleaning mode to becarried out.

Preferably, a decrease in the electrical resistance between at least twoelectrode layers indicates that particles, in particular soot particles,have been deposited on or between the electrodes (electrode layers). Assoon as the electrical resistance reaches a lower threshold value, theactivation of the heating coil or heating layer takes place. Theparticles are in other words burned off. With an increasing number ofburnt-off particles or burnt-off particle volume, the electricalresistance increases. The burning off is preferably carried out for sucha time until an upper electrical resistance value is measured. Reachingan upper electrical resistance value is taken as an indication of aregenerated or cleaned sensor. A new measuring cycle can subsequentlybegin or be carried out.

Alternatively or in addition, it is possible to measure a change in thecapacitances of the electrode layers. An increasing loading of thearrangement of electrode layers leads to an increase in the capacitanceof the electrode layers. The arrangement of particles, in particularsoot particles, in at least one passage of the sensor leads to a chargetransfer or a change in the permittivity (s), which leads to an increasein the capacitance (C). In principle: C=(ε×A)/d, where A stands for theactive electrode area of the electrode layer and d stands for thedistance between two electrode layers.

The measuring of the capacitance may be carried out by way of exampleby:

-   determining the rate of voltage increase with a constant current    and/or-   applying a voltage and determining the charging current and/or-   applying an AC voltage and measuring the current profile and/or-   determining the resonant frequency by means of an LC oscillating    circuit.

The described measurement of the change in the capacitances of theelectrode layers may also be carried out in connection with a monitoringmode to be carried out.

According to OBD (on-board diagnosis) regulations, all parts andcomponents that are relevant to exhaust gas must be checked for theirfunction. The functional check is to be carried out for example directlyafter starting a motor vehicle.

For example, at least one electrode layer may be destroyed, this beingaccompanied by a reduction in the active electrode area A. Since theactive electrode area A is directly proportional to capacitance C, themeasured capacitance C of a destroyed electrode layer decreases.

In the monitoring mode, it is alternatively or additionally possible toform the electrode layers as conductor circuits. The conductor circuitsmay be formed as closed or open conductor circuits, which can be closedon demand, for example by a switch. It is also possible to close theelectrode layers by way of at least one switch to form at least oneconductor circuit, it being checked in the monitoring mode whether atest current is flowing through the at least one conductor circuit. Ifan electrode layer has a crack or is damaged or destroyed, no testcurrent would flow.

According to an independent aspect, the invention relates to a methodfor producing a sensor for detecting electrically conductive and/orpolarizable particles, in particular a method for producing a describedsensor according to the invention.

The method comprises that a laminate with a first electrode layer, asecond electrode layer, a first insulation layer, which is arrangedbetween the first electrode layer and the second electrode layer,optionally at least a third electrode layer, which is arranged betweenthe first insulation layer and the first electrode layer, and optionallyat least a second insulation layer, which is arranged between the thirdelectrode layer and the first electrode layer, is produced, at least onepassage that extends over the first electrode layer, the optionally atleast second insulation layer, the optionally at least third electrodelayer, and the first insulation layer being subsequently introduced intothe laminate, the bottom of the passage being formed by a portion of thesecond electrode layer.

The method is also based on the idea of producing a laminate whichcomprises at least three electrode layers and two insulation layers, inorder to introduce at least one passage into this laminate. The passageserves as access to the second electrode layer for the particles to bedetected, in particular soot particles.

The production of the laminate and/or of the individual layers of thelaminate may take place by a thin-film technique or a thick-filmtechnique or a combination of these techniques. As part of a thin-filmtechnique to be applied, a vapor depositing process or preferably acathode sputtering process may be chosen. As part of a thick-filmprocess, a screen-printing process is conceivable in particular.

At least one insulation layer and/or at least one covering layer, whichis formed on the side of the first electrode layer that is facing awayfrom the first insulation layer, may be formed by a chemical vapordeposition (CVD process) or a plasma-enhanced chemical vapor deposition(PECVD process).

The first insulation layer may be produced in such a way that itlaterally encloses the second electrode layer. An optionally presentcovering layer may likewise be produced in such a way that it laterallyencloses the first electrode layer and/or the at least second insulationlayer and/or the at least third electrode layer and/or the firstinsulation layer and/or the second electrode layer. Accordingly, both atleast one of the insulation layers and at least one/the covering layermay form an additional lateral enclosure.

The passage may for example be formed as a blind hole or as an elongatedepression, the at least one blind hole or a subportion of the blindhole or the at least one elongate depression or a subportion of theelongate depression being introduced into the laminate by at least oneremoving or etching process, in particular by a plasma-ion etchingprocess, or by a number of successively carried out removing or etchingprocesses which is adapted to the layer of the laminate that isrespectively to be etched or to be removed.

In other words, a blind hole or an elongate depression may be introducedinto the laminate in such a way that, for example for each layer to bepenetrated or to be etched or to be removed, a process that is optimumfor this layer is used, and consequently a number of etching or removingsteps that are to be successively carried out are carried out.

It is also conceivable that the blind hole or a subportion of the blindhole or the elongate depression or a subportion of the elongatedepression may be made in a chemical etching process from the liquid orvapor phase. The first electrode layer preferably consists of a metal,in particular a platinum layer, which is relatively easy to etch throughor to etch.

In one possible embodiment of the method according to the invention itis possible that the etching process stops at the second electrode layerif the second electrode layer is produced from a material that is moreresistant to etching in comparison with the first and third electrodelayers and with the insulation layers. If the laminate or the sensorcomprises an additional covering layer, the second electrode layer alsocomprises a material that is more resistant to etching in comparisonwith this covering layer. For example, the second electrode layer isproduced from a platinum-titanium alloy (Pt/Ti). It is also conceivablethat the second electrode layer consists of a layer filled with metaloxides.

In a further embodiment of the method according to the invention it ispossible that the first insulation layer and/or the at least secondinsulation layer is formed as a layer stopping the etching process and,in a further step, a subportion of the blind hole or a subportion of theelongate depression is introduced into the first insulation layer and/orthe at least second insulation layer by a conditioning process or aconditioning step with phase conversion of the first insulation layerand/or the at least second insulation layer.

In a further embodiment of the method according to the invention it ispossible that the at least one passage and/or a passage is formed as ablind hole or as an elongate depression and this blind hole or the atleast one blind hole or a subportion of the blind hole or this elongatedepression or the at least one elongate depression or a subportion ofthe elongate depression is introduced into the laminate by a process ofirradiating with electromagnetic waves or charged particles (electrons),the radiation source and/or the wavelength and/or the pulse frequency ofthe radiation being adapted to the layer of the laminate that isrespectively to be machined.

It is preferably possible that the at least one passage and/or a passageis formed as a blind hole or as an elongate depression and this blindhole or the at least one blind hole or a subportion of the blind hole orthis elongate depression or the at least one elongate depression or asubportion of the elongate depression is introduced into the laminate bya laser machining process, in particular by means of an ultrashort pulselaser, the laser source and/or the wavelength and/or the pulse frequencyof the laser and/or the energy of the charged particles and/or thespecies of the charged particles being adapted to the layer of thelaminate that is respectively to be machined. Particularly preferably,an ultrashort pulse laser is a femto laser or a pico laser.

One possibility for producing the passage that is formed as a blind holeor as an elongate depression is consequently the partial removal of thelaminate by means of a laser. Laser sources with different wavelengthsand/or pulse frequencies that are respectively made to suit the materialto be removed can be used. Such a procedure has the advantage that, bymaking them suit the material of the layer that is to be removed, therespectively individual laser machining steps can be carried outquickly, so that overall an improved introduction of passages and/orblind holes and/or elongate depressions into the laminate is obtained.The use of an ultrashort pulse laser proves to be particularlyadvantageous.

Apart from electromagnetic radiation, charged or uncharged particles canhowever also be used for removing the electrode layers and/or insulationlayers. Thus, apart from electron beams, other charged or unchargedparticles can also be used for the ablation. This may be carried outwith or without masks that contain the structural information to betransferred.

In a further embodiment of the method according to the invention it ispossible that, when producing the laminate, the first insulation layerand/or the at least second insulation layer is created over the fullsurface area, in particular by a screen-printing process or spraying-onprocess or immersion process or spin-coating process, between the secondelectrode area and the at least third electrode area or between the atleast third electrode area and the first electrode area and, in asubsequent method step, at least a portion of the first insulation layerand/or of the at least second insulation layer is removed, in particularby structured dissolving or etching or burning out, in such a way thatthe passage is formed in the sensor.

Such a method corresponds to the lost mold principle. Accordingly, it ispossible, especially in the case of thermally stable materials, toperform structuring by the lost mold principle. A lost mold serves forcreating a passage from the first electrode layer to the secondelectrode layer. The at least one insulation layer or insulating layeris created between the electrode layers from a thermally stablematerial, a portion of this insulation layer preferably being removed bydissolving or etching or burning out after the application of the firstelectrode layer. As a result of this, the first electrode layer locatedthereover is also removed. If a covering layer is formed, the portion ofthe covering layer that is located over the removed portion of theinsulation layer is also removed by the dissolving or etching or burningout of the portion of the insulation layer.

Preferably, after the introduction of a passage and/or a blind holeand/or an elongate depression into the laminate, at least one passiveporous filter layer is applied on the covering layer. The passive porousfilter layer is formed for example by an aluminum oxide foam. This isalso formed over the at least one passage or over the at least one blindhole or over the at least one elongate depression.

In a further independent aspect, the invention relates to a method thatserves for producing a sensor for detecting electrically conductiveparticles and/or polarizable particles.

A laminate with a first electrode layer, a second electrode layer, afirst insulation layer, which is arranged between the first electrodelayer and the second electrode layer, at least a third electrode layer,which is arranged between the first insulation layer and the firstelectrode layer, and at least a second insulation layer, which isarranged between the third electrode layer and the first electrodelayer, is produced, the first insulation layer, the at least thirdelectrode layer, the at least second insulation layer and the firstelectrode layer being formed as porous layers. The pores in the firstand third electrode layer and the first and second insulation layer areset in such a way that at least one pore in the first electrode layer,at least one pore in the at least second insulation layer, at least onepore in the at least third electrode layer and at least one pore in thefirst insulation layer are arranged at least in certain portions oneover the other, so that at least one passage to the second electrodelayer is produced.

If the sensor has a covering layer, this covering layer is also appliedto the first electrode layer with a pore size and porosity, at least onepore in the covering layer being arranged at least in certain portionsover a pore in the first electrode layer, over a pore in the at leastsecond insulation layer, over a pore in the at least third electrodelayer and a pore in the first insulation layer in such a way that,starting from the covering layer, at least one passage to the secondelectrode layer is formed. A passive porous filter layer may finally beapplied to the covering layer.

In a further independent aspect, a method for producing a sensor fordetecting electrically conductive particles and/or polarizable particlesis provided, a laminate with a first electrode layer, a second electrodelayer, at least a first insulation layer, which is arranged between thefirst electrode layer and the second electrode layer, optionally atleast a third electrode layer, which is arranged between the firstinsulation layer and the first electrode layer, and optionally at leasta second insulation layer, which is arranged between the third electrodelayer and the first electrode layer, being produced, the firstinsulation layer, the at least third electrode layer, the at leastsecond insulation layer and the first electrode layer being structured,in particular created by a lift-off process and/or an ink-jet processand/or in a stamping process, in such a way that, as a result of thestructured application of the individual layers one over the other, apassage to the second electrode layer is formed.

In other words, already during the production of the insulation layer(s)and/or the first and/or third electrode layer, such a structure that hasopenings or clearances is produced, a number of openings that arearranged at least in certain portions one over the other forming atleast one passage to the second electrode layer. If the sensor has acovering layer, this covering layer may also be applied in an alreadystructured form to the first electrode layer.

In the case of all of the described processes for producing a sensor fordetecting electrically conductive and/or polarizable particles, it isnecessary that an electrical contacting area is respectively formed inthe first electrode layer and/or in the second electrode layer and/or inthe optionally at least third electrode layer. This is achieved byportions of the first electrode layer and/or of the second electrodelayer and/or of the optionally at least third electrode layer being keptfree from sensor layers arranged over the respective electrode layers.This may take place on the one hand by the electrical contacting areasbeing produced by removing and/or etching away and/or lasering awaysensor layers arranged thereover. It is also conceivable that theinsulation layers and/or the electrode layers and/or the coveringlayer(s) are applied to one another in a structured form, so that theelectrical contacting areas are already kept free during the productionof the individual sensor layers.

As an alternative or in addition, it is possible that at least theinsulation layers, preferably all of the layers, of the laminate of thesensor are produced by means of an HTCC (high temperature cofiredceramics) process. The insulation layers are produced by combiningpowder, for example ceramic powder, metal powder, aluminum oxide powderand glass powder, and also an amount of binder and solvent, whichtogether form a homogeneous liquid mass. This mass is applied to a filmstrip, so that green sheets are formed. The drying of the green sheetssubsequently takes place. The dried green sheets may be cut and/orpunched and/or shaped, in particular provided with openings.Subsequently, the green sheets may for example be rolled up andtransported for further processing.

The electrode layers may for example be produced on the green sheet byprinting, in particular by screen printing or stencil printing, frommetal pastes. Alternatively, thin metal films may be produced andcorrespondingly prestructured.

Once the various substrate, electrode and insulation layers have beencreated, the green sheets are arranged in the desired sequence andpositioned in exact register one over the other, pressed and joinedtogether by thermal treatment. The binder may be of an organic orinorganic nature and during the thermal treatment either turns into astable material or combusts or evaporates. The particles thereby fusefirmly to one another by melting and/or sintering processes during thethermal treatment. In this way, the three-dimensional structure of thesensor is formed or produced.

In a further embodiment of the invention it is conceivable that, whenproducing the laminate, the electrical contacting areas are covered withthe aid of stencils, so that the electrical contacting areas cannot becoated with other sensor layers.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail below on the basis ofexemplary embodiments with reference to the accompanying schematicdrawings, in which:

FIGS. 1a-c show sectional representations of various embodiments ofsensors for detecting electrically conductive and/or polarizableparticles;

FIG. 2 shows a perspective plan view of a sensor according to theinvention;

FIG. 3 shows a possible formation of a second electrode layer;

FIG. 4 shows a sectional representation of a further embodiment of asensor for detecting electrically conductive and/or polarizableparticles;

FIG. 5 shows a sectional representation of a further embodiment of asensor for detecting electrically conductive and/or polarizableparticles which comprises at least three electrode layers;

FIGS. 6a-f show representations of various embodiments of openings;

FIGS. 7 a+b show representation of a possible arrangement of a sensor ina fluid flow;

FIGS. 8 a+b show representations of various cross sections orcross-sectional profiles of passages;

FIG. 9 shows a sectional representation of undercuts in insulationlayers or set-back insulation layers; and

FIGS. 10a-d show exploded representations of various embodiments of asensor according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The same reference numerals are used below for parts that are the sameand parts that act in the same way.

FIG. 1 a shows in a sectional representation a sensor 10 for detectingelectrically conductive and/or polarizable particles, in particular fordetecting soot particles. The sensor 10 comprises a substrate 11, afirst electrode layer 12 and a second electrode layer 13, which isarranged between the substrate 11 and the first electrode layer 12. Aninsulation layer 14 is formed between the first electrode layer 12 andthe second electrode layer 13. At least one opening is respectivelyformed in the first electrode layer 12 and in the insulation layer 14,the opening 15 in the first electrode layer 12 and the opening 16 in theinsulation layer 14 being arranged one over the other, so that a passage17 to the second electrode layer 13 is formed.

For the purposes of a high-temperature application, the substrate 11 isformed for example from aluminum oxide (Al₂O₃) or magnesium oxide (MgO)or from a titanate or from steatite.

The second electrode layer 13 is connected to the substrate 11indirectly by way of a bonding agent layer 18. The bonding agent layer18 may be for example very thinly formed aluminum oxide (Al₂O₃) orsilicon dioxide (SiO₂).

In the exemplary embodiment, the first electrode layer 12 is formed by aplatinum layer. In the example shown, the second electrode layer 13consists of a platinum-titanium alloy (Pt—Ti). The platinum-titaniumalloy of the second electrode layer 13 is a layer that is more resistantto etching in comparison with the first electrode layer 12.

The distance between the first electrode layer 12 and the secondelectrode layer 13 is formed by the thickness d of the insulation layer14. The thickness d of the insulation layer may be 0.5 μm to 50 μm. Inthe present case, the thickness d of the insulation layer is 10 μm. Thesensitivity of the sensor 10 according to the invention can be increasedby reducing the distance between the first electrode layer 12 and thesecond electrode layer 13, and consequently by reducing the thickness dof the insulation layer 14.

The insulation layer 14 covers the second electrode layer 13 on the sideface 19 shown, so that the second electrode layer 13 is laterallyenclosed and insulated.

The passage 17 is formed as a blind hole, a portion of the secondelectrode layer 13 being formed as the bottom 28 of the blind hole. Theblind hole or the passage 17 extends over the insulation layer 14 andover the first electrode layer 13. The passage 17 is in other wordsformed by the openings 15 and 16 arranged one over the other. In theembodiment shown, the openings 15 and 16 are not formed peripherally.

A soot particle 30 can enter the passage 17. In FIG. 1 a, the particle30 is lying on the bottom 28 of the blind hole, and consequently on aside 31 of the second electrode layer 13. However, the particle 30 isnot touching the first electrode layer 12 in the peripheral region 32,which bounds the opening 15. As a result of the particle 30 beingdeposited on the bottom 28 and touching the second electrode layer 13 onthe side 31, the electrical resistance is reduced. This drop in theresistance is used as a measure of the accumulated mass of particles.When a predefined threshold value with respect to the resistance isreached, the sensor 10 is heated, so that the deposited particle 30 isburned and, after being burned free, the sensor 10 can detectelectrically conductive and/or polarizable particles in a next detectioncycle.

FIG. 1b likewise shows in a sectional representation a sensor 10 fordetecting electrically conductive and/or polarizable particles, inparticular for detecting soot particles. Likewise shown are a firstelectrode layer 12 and a second electrode layer 13, which is arrangedbetween the substrate 11 and the first electrode layer 12. An insulationlayer 14 is formed between the first electrode layer 12 and the secondelectrode layer 13. With respect to the properties and the design of theopenings 15 and 16, reference is made to the explanations in connectionwith the embodiment according to FIG. 1 a.

A covering layer 21, which is for example formed from ceramic and/orglass and/or metal oxide, is formed on the side 20 of the firstelectrode layer 12 that is facing away from the insulation layer 14. Thecovering layer 21 encloses the side face 22 of the first electrode layer12, the side face 23 of the insulation layer 14 and the side face 19 ofthe second electrode layer 13. The covering layer 21 consequently coversthe side faces 19, 22 and 23, so that the first electrode layer 12, thesecond electrode layer 13 and the insulation layer 14 are laterallyinsulated. The covering layer 21 consequently comprises an upper portion24, which is formed on the side 20 of the first electrode layer 12, anda side portion 25, which serves for the lateral insulation of the sensor10.

FIG. 1c shows in a sectional representation a sensor 10 for detectingelectrically conductive and/or polarizable particles, in particular fordetecting soot particles. The sensor 10 comprises a substrate 11, afirst electrode layer 12 and a second electrode layer 13, which isarranged between the substrate 11 and the first electrode layer 12. Aninsulation layer 14 is formed between the first electrode layer 12 andthe second electrode layer 13. At least one opening is respectivelyformed in the first electrode layer 12 and in the insulation layer 14,the opening 15 in the first electrode layer 12 and the opening 16 in theinsulation layer 14 being arranged one over the other, so that a passage17 to the second electrode layer 13 is formed.

For the purposes of a high-temperature application, the substrate 11 isformed for example from aluminum oxide (Al₂O₃) or magnesium oxide (MgO)or from a titanate or from steatite.

The second electrode layer 13 is connected to the substrate 11indirectly by way of a bonding agent layer 18. The bonding agent layer18 may be for example very thinly formed aluminum oxide (Al₂O₃) orsilicon dioxide (SiO₂).

In the exemplary embodiment, the first electrode layer 12 is formed by aplatinum layer. In the example shown, the second electrode layer 13consists of a platinum-titanium alloy (Pt—Ti). The platinum-titaniumalloy of the second electrode layer 13 is a layer that is more resistantto etching in comparison with the first electrode layer 12.

The insulation layer 14 consists of a thermally stable material with ahigh insulation resistance. For example, the insulation layer 14 may beformed from aluminum oxide (Al₂O₃) or silicon dioxide (SiO₂) ormagnesium oxide (MgO) or silicon nitride (Si₃N₄) or glass.

The distance between the first electrode layer 12 and the secondelectrode layer 13 is formed by the thickness d of the insulation layer14. The thickness d of the insulation layer may be 0.5 μm to 50 μm. Inthe present case, the thickness d of the insulation layer is 10 μm. Thesensitivity of the sensor 10 according to the invention can be increasedby reducing the distance between the first electrode layer 12 and thesecond electrode layer 13, and consequently by reducing the thickness dof the insulation layer 14.

A covering layer 21, which is for example formed from ceramic and/orglass and/or metal oxide, is formed on the side 20 of the firstelectrode layer 12 that is facing away from the insulation layer 14. Thecovering layer 21 encloses the side face 22 of the first electrode layer12, the side face 23 of the insulation layer 14 and the side face 19 ofthe second electrode layer 13. The covering layer 21 consequently coversthe side faces 19, 22 and 23, so that the first electrode layer 12, thesecond electrode layer 13 and the insulation layer 14 are laterallyinsulated. The covering layer 21 consequently comprises an upper portion24, which is formed on the side 20 of the first electrode layer 12, anda side portion 25, which serves for the lateral insulation of the sensor10.

In a further embodiment of the invention it is conceivable that thecovering layer 21 also laterally encloses the substrate 11.

A porous filter layer 27 is formed on the side 26 of the covering layer21 that is facing away from the first electrode layer 12. Thesensitivity of the sensor 10 is increased as a result of the formationof this passive porous filter or protective layer 27 which is facing themedium that is to be detected with regard to electrically conductiveand/or polarizable particles, since larger particles or constituentsthat could disturb the measurement or detection are kept away from thefirst electrode layer 12 and the second electrode layer 13. Since thepassage 17 is covered by the porous filter layer 27, particles can stillpenetrate through the pores in the porous filter layer 27, butshort-circuits caused by large penetrated particles can be avoided as aresult of the porous filter layer 27.

The passage 17 is formed as a blind hole, a portion of the secondelectrode layer 13 being formed as the bottom 28 of the blind hole. Theblind hole or the passage 17 extends over the insulation layer 14, thefirst electrode layer 13 and over the covering layer 21. For thispurpose, the covering layer 21 also has an opening 29. In other words,the passage 17 is formed by the openings 29, 15 and 16 arranged one overthe other.

As a result of the choice of materials for the individual layers and theinsulation of the individual layers from one another, the sensor 10shown is suitable for a high-temperature application of up to forexample 850° C. The sensor 10 can accordingly be used as a soot particlesensor in the exhaust-gas flow of an internal combustion engine.

After penetrating through the porous filter layer 27, a soot particle 30can enter the passage 17. In FIG. 1 c, the particle 30 lies on thebottom 28 of the blind hole, and consequently on a side 31 of the secondelectrode layer 13. However, the particle is not touching the firstelectrode layer 12 in the peripheral region 32, which bounds the opening15. As a result of the particle 30 being deposited on the bottom 28 andtouching the second electrode layer 13 on the side 31, the electricalresistance is reduced. This drop in the resistance is used as a measureof the accumulated mass of particles. When a predefined threshold valuewith respect to the resistance is reached, the sensor 10 is heated, sothat the deposited particle 30 is burned and, after being burned free,the sensor 10 can detect electrically conductive and/or polarizableparticles in a next detection cycle.

FIG. 2 shows a perspective view of a sensor 10. The sensor has ninepassages 17. For better illustration, the porous filter layer 27 is notshown in FIG. 2. The upper portion 24 of the covering layer 21 and alsothe side portion 25 of the covering layer 21 can be seen. The bottoms 28of the passages 17 are formed by portions of the second electrode layer13. The nine passages 17 have a square cross section, it being possiblefor the square cross section to have a surface area of 15×15 μm² to50×50 μm².

The first electrode layer 12 has an electrical contacting area 33. Thesecond electrode layer 13 likewise has an electrical contacting area 34.The two electrical contacting areas 33 and 34 are free from sensorlayers arranged over the respective electrode layers 12 and 13. Theelectrical contacting areas 33 and 34 are or can in each case beconnected to a terminal pad (not shown).

The second electrode layer 13 has an additional electrical contactingarea 35, which is likewise free from sensor layers arranged over theelectrode layer 13. This additional electrical contacting area 35 may beconnected to an additional terminal pad. The additional electricalcontacting area 35 is necessary to allow the second electrode layer 13to be used as a heating coil or as a temperature-sensitive layer or as ashielding electrode. Depending on the contacting assignment (see FIG. 3)of the electrical contacting areas 34 and 35, the second electrode layer13 may either heat and burn the particle 30 or detect the particle 30.

To be able to use an electrode layer, here the second electrode layer13, as a heating coil and/or temperature-sensitive layer and/orshielding electrode, the second electrode layer 13 has a small number ofstrip conductor loops 36.

In FIG. 4, a further embodiment of a possible sensor 10 is shown. Thefirst electrode layer 12 and the insulation layer 14 are respectivelyformed as porous, the at least one opening 15 in the first electrodelayer 12 and the at least one opening 16 in the insulation layer 14respectively being formed by at least one pore, the pore 41 in theinsulation layer 14 and the pore 40 in the first electrode layer 12being arranged at least in certain portions one over the other in such away that the at least one passage 17 to the second electrode layer 13 isformed. In other words, it is possible to dispense with an active orsubsequent structuring of the passages, the first electrode layer 12 andthe insulation layer 14 being formed as permeable to the medium to bemeasured. The passages 17 are represented in FIG. 4 with the aid of thevertical arrows.

The passages 17 may be formed by a porous or granular structure of thetwo layers 12 and 14. Both the first electrode layer 12 and theinsulation layer 14 can be produced by sintering together individualparticles, with pores 40 and 41 or voids for the medium to be measuredbeing formed while they are being sintered together. Accordingly, apassage 17 that allows access to the second electrode layer 13 for aparticle 30 that is to be measured or detected must be formed, extendingfrom the side 20 of the first electrode layer 12 that is facing awayfrom the insulation layer 14 to the side 31 of the second electrodelayer 13 that is facing the insulation layer 14 as a result of theone-over-the-other arrangement of pores 40 and 41 in the first electrodelayer 12 and in the insulation layer 14.

In the example shown, the second electrode layer 13 is completelyenclosed on the side face 19 by the porous insulation layer 14. Thesecond electrode layer 13 is accordingly covered on the side 31 and onthe side faces 19 by the porous insulation layer 14. The porous firstelectrode layer 12 on the other hand encloses the porous insulationlayer 14 on the side face 23 and on the side 37 facing away from thesecond electrode layer 13. The insulation layer 14 is accordinglycovered on the side 37 and on the side faces 23 by the first electrodelayer 12.

If this sensor 10 has a covering layer, this covering layer is also tobe formed as porous in such a way that a pore in the covering layer, apore 40 in the first electrode layer 12 and a pore 41 in the insulationlayer 14 form a passage 17 to the second electrode layer 13.

In FIG. 5, a section through a sensor 10 for detecting electricallyconductive and/or polarizable particles, in particular for detectingsoot particles, is shown. The sensor 10 can in principle be used fordetecting particles in gases and in liquids. The sensor 10 comprises asubstrate 11, a first electrode layer 12, a second electrode layer 13,which is arranged between the substrate 11 and the first electrode layer12, a first insulation layer 14 being formed between the first electrodelayer 12 and the second electrode layer 13.

At least a third electrode layer 50 is formed between the firstinsulation layer 14 and the first electrode layer 12, at least a secondinsulation layer 60 being formed between the third electrode layer 50and the first electrode layer 12.

According to sensor 10 of FIG. 5, therefore at least three electrodelayers 12, 13, 50 and at least two insulation layers 14, 60 are formed.The first electrode layer 12 is in this case the electrode layer that isarranged furthest away from the substrate 11. The second electrode layer13 on the other hand is connected directly to the substrate 11. It ispossible that the second electrode layer 13 is connected indirectly tothe substrate 11, preferably by means of a bonding agent layer.

In the embodiment according to FIG. 5, a fourth electrode layer 51 isalso formed and also a third insulation layer 61. The sensor 10consequently comprises altogether four electrode layers, to be specificthe first electrode layer 12, the second electrode layer 13, and alsothe third electrode layer 50 and the fourth electrode layer 51.Insulation layers are respectively formed between the electrode layers(12, 13, 50, 51), to be specific the first insulation layer 14, thesecond insulation layer 60 and also the third insulation layer 61. Thesensor 10 also comprises a covering layer 21, which is formed on theside of the first electrode layer 12 that is facing away from thesubstrate 11.

At least one opening 15, 16, 70, 71, 72, 73 is respectively formed inthe first electrode layer 12, in the third insulation layer 61, in thefourth electrode layer 51, in the second insulation layer 60, in thethird electrode layer 50 and in the first insulation layer 14. Thecovering layer 21 also has an opening 29. The opening 15 in the firstelectrode layer 12, the opening 73 in the third insulation layer 61, theopening 72 in the fourth electrode layer 51, the opening 71 in thesecond insulation layer 60, the opening 70 in the third electrode layer50 and the opening 16 in the first insulation layer 14 are arranged atleast in certain portions one over the other in such a way that at leastone passage 17 to the second electrode layer 13 is formed.

The distance between the electrode layers 12, 13, 50 and 51 is formed bythe thickness of the insulation layers 14, 60 and 61. The thickness ofthe insulation layers 14, 60 and 61 may be 0.1 μm to 50 μm. Thesensitivity of the sensor 10 according to the invention can be increasedby reducing the distance between the electrode layers 12, 13, 50 and 51,and consequently by reducing the thickness of the insulation layers 14,60 and 61.

The passage 17 is formed as a blind hole, a portion of the secondelectrode layer 13 being formed as the bottom 28 of the blind hole. Theblind hole or the passage 17 extends over the first insulation layer 14,the third electrode layer 50, the second insulation layer 60, the fourthelectrode layer 51, the third insulation layer 61, the first electrodelayer 12 and over the covering layer 21. In other words, the passage 17is formed by the openings 16, 70, 71, 72, 73, 15 and 29 arranged overone another. In the embodiment shown, the openings 16, 70, 71, 72, 73,15 and 29 are not formed peripherally. A perspective section through apassage 17 is shown.

A small soot particle 30 for example can enter the passage 17. In FIG.5, the particle 30 is lying on the bottom 28 of the blind hole, andconsequently on a side 31 of the second electrode layer 13. The particle30 is also touching the third electrode layer 50. If the determinationof particles is performed on the basis of the resistive principle, theresistance between the second electrode layer 13 and the third electrodelayer 50 is measured, this resistance decreasing if the particle 30bridges the two electrode layers 13 and 50. The size of the particle 30is consequently relatively small.

The soot particle 30′ has also entered the passage 17. The particle 30′is lying on the bottom 28 of the blind hole, and consequently on theside 31 of the second electrode layer. The particle 30′ is also touchingthe third electrode layer 50, the fourth electrode layer 51 and also thefirst electrode layer 12. The particle 30′ consequently bridges a numberof electrode layers, in the example shown all of the electrode layers12, 13, 50 and 51, so that the particle 30′ is detected as a particlethat is larger in comparison with the particle 30.

By applying different voltages to the electrode layers 12, 13, 50 and51, different particle properties, in particular different sootproperties, such as for example the diameter and/or the size of the(soot) particle and/or the charging of the (soot) particle and/or thepolarizability of the (soot) particle, can be measured.

Various embodiments of openings 80 are shown in FIGS. 6a to 6 f. Theopenings 80 may be formed both in insulation layers 14, 60 and 61 and inelectrode layers 12, 50 and 51. Accordingly, the openings 80 that areshown may be an arrangement of openings 15 in a first electrode layer12, openings 16 in a first insulation layer 14, openings 70 in a thirdelectrode layer 50, openings 71 in a second insulation layer 60,openings 72 in a fourth electrode layer 51 and also openings 73 in athird insulation layer 61.

Preferably, the openings 80 in a laminate of the sensor 10 are formedsimilarly. The individual layers 12, 14, 21, 50, 51, 60 and 61 arearranged one over the other in such a way that the openings 15, 16, 29,70, 71, 72 and 73 form passages 17. As a result of the openings shown inFIGS. 6a to 6 d, elongate depressions 17′ and 17″ are respectivelyformed.

In FIG. 6 a, linear openings 80 are formed, the openings 80 being formedparallel to one another and all pointing in the same predominantdirection.

In FIG. 6 b, a layer of the sensor 10 is subdivided into a first portion45 and a second portion 46. All of the openings 80, 80′ shown are formedas linear clearances, with both the openings 80 in the first portion 45being formed parallel to one another, and the openings 80′ in the secondportion 46 being formed parallel to one another. The openings 80 in thefirst portion 45 run parallel in the horizontal direction or parallel tothe width b of the sensor layer, whereas the openings 80′ in the secondportion 46 run parallel in the vertical direction or parallel to thelength l of the sensor layer. The openings 80′ in the second portion 46run in a perpendicular direction in relation to the openings 80 in thefirst portion 45.

In FIG. 6 c, likewise a number of openings 80, 80′, 80″ are shown in theform of elongate clearances. In a central portion 47, a number of linearopenings 80′ running in the vertical direction are shown, in the exampleshown eight openings, which are formed parallel to the length l of thesensor layer. These openings are surrounded by further openings 80, 80″,forming a frame-like portion 48. First openings 80″ are in this caseformed parallel to the openings 80′ of the central portion 47. Furtheropenings 80 are formed perpendicularly in relation to the openings 80,80″. The openings 80″ are of different lengths, so that the layer of thesensor 10 can be formed with a largest possible number of openings 80.

In FIG. 6 d, a sensor layer with an elongate through-opening 80 isshown, the opening 80 running in a meandering manner.

In FIG. 6 e, a further sensor layer with a number of vertically runningopenings 80′ and a number of horizontally running openings 80 is shown.The vertical openings 80′ and the horizontal openings 80 form a gridstructure.

Apart from rectangular grid structures, other angular arrangements canalso be produced, or geometries in which the grid or network structurehas round, circular or oval shapes. Furthermore, correspondingcombinations of the structures, which may be regular, periodic orirregular, can be created.

In FIG. 6 f, a sensor layer with an elongate through-opening 80 isshown, the opening 80 running spirally. Apart from rectangulargeometries, circular, oval geometries or combinations thereof can alsobe produced.

In each case a number of layers, which respectively have openings 80,80′, 80″ according to an embodiment of FIG. 6 a, 6 b, 6 c, 6 d, 6 e or 6f, are arranged in layers one over the other, so that passages in theform of elongate depressions 17′ and 17″ are respectively formed in asensor.

As shown in FIG. 7 a, a sensor 10 is introduced into a fluid flow insuch a way that the direction of flow a of the particles does notimpinge perpendicularly on the plane (x, y) of the electrode layers. Theangle α between the normal (z) to the plane (x, y) of the firstelectrode layer and the direction of flow of the particles is in thiscase at least 1 degree, preferably at least 10 degrees, particularlypreferably at least 30 degrees. The particles can consequently be guidedmore easily into the elongate depressions 17′, 17″, and consequentlymore easily to the walls of the openings of the electrode layers 12, 50,51 formed therein.

In FIG. 7 b, a sensor 10 has thus been introduced into a fluid flow insuch a way that the angle β between the direction of flow a of theparticles and the longitudinal axis x of the elongate depressions liesbetween 20 and 90 degrees.

In FIGS. 8a and 8 b, a cross section which is taken perpendicularly tothe sensor 10, that is to say beginning from the uppermost insulation orcovering layer 21 to the substrate 11, is respectively shown. Thesensors 10 of FIGS. 8a and 8b have four electrode layers, to be specifica first electrode layer 12, a second electrode layer 13 and also a thirdelectrode layer 50 and a fourth electrode layer 51. Also formed arethree insulation layers, to be specific a first insulation layer 14, asecond insulation layer 60 and also a third insulation layer 61.

In the sensor 10 according to FIG. 8 a, the cross-sectional profiles oftwo passages in the form of elongate depressions 17′, 17″ are shown. Theleft passage 17′ has a V-shaped cross section or a V-shapedcross-sectional profile. The right passage 17″ on the other hand has aU-shaped cross section or a U-shaped cross-sectional profile. The sizesof the openings or cross sections of the openings decrease from thecovering layer 21 in the direction of the second electrode layer 13. Thecross sections of the openings 29, 15, 73, 72, 71, 70 and 16 becomeincreasingly smaller from the first cross section of an opening 29 inthe direction of the lowermost cross-sectional opening 16.

With the aid of the V-shaped and U-shaped cross-sectional profiles, themeasurements of round particles are improved.

In FIG. 8b it is also shown that the passages 17′, 17″ can havedifferent widths. The left passage 17′ has a width B1. The right passage17″ shown has a width B2. B1 is greater than B2. As a result of passages17′, 17″ formed with different widths, size-specific measurements of theparticles 30 can be carried out.

In FIG. 9, undercuts in insulation layers 14, 21, 60, 61 or set-backinsulation layers 14, 21, 60, 61 are shown in cross section. In the caseof round particles, the formation of level or smooth passage surfaces isunfavorable. The measurement of round particles can be improved by theformation of undercuts or set-back insulation layers.

The left passage 17′ shown has a first insulation layer 14, a secondinsulation layer 60 and also a third insulation layer 61 and a coveringlayer 21, which also serves as an insulation layer. The insulationlayers 14, 60, 61 and 21 have undercuts or clearances 90. The size ofthe openings 16, 71, 73 and 29 in the insulation layers 14, 60, 61 and21 are consequently greater than the openings 70, 72 and 15 in theelectrode layers 12, 50 and 51 that are respectively formed over andunder the insulation layers 14, 60, 61 and 21.

This also applies in connection with the passage 17″ shown on the right.In this case, the insulation layers 14, 16, 61 and 21 are formed asset-back in comparison with the electrode layers 50, 51 and 12. Theopenings 16, 71 or 73 in an insulation layer 14, 60 or 61 is formedlarger in each case than an opening 70, 72 or 15 formed thereover in anelectrode layer 50, 51 or 12 arranged over the respective insulationlayer. Since the cross-sectional profile of the right passage 17″ isformed in a V-shaped manner and the openings in all the layers 21, 12,61, 51, 60, 50 and 14 become smaller in the direction of the substrate11, the openings 16, 71, 73 and 29 in the insulation layers 14, 60, 61and 21 are not of coinciding sizes.

It should be pointed out in connection with the sensors 10 shown inFIGS. 5, 8 a, 8 b and 9 that it is possible that only two uppermostelectrode layers have to be made accessible within a passage. In otherwords, in a method, preferably according to the invention, a passage 17,17′, 17″ that is merely formed with respect to the uppermost electrodelayers 12 and 51 may be formed in a sensor 10.

It is also possible that a sensor 10 comprises a number of passages 17,17′, 17″, at least a first passage merely reaching as far as the fourthelectrode layer 51. The fourth electrode layer 51 or the secondinsulation layer 60 forms the bottom of this passage formed.

A second passage reaches as far as the third electrode layer 50. Thethird electrode layer 50 or the first insulation layer 14 forms thebottom of the passage formed. A third passage reaches as far as thesecond electrode layer 13. The second electrode layer 13 forms thebottom of the passage formed.

This embodiment can be carried out or can be formed independently of thefeatures of the sensors 10 shown in FIGS. 5, 8 a, 8 b and 9.

The exploded representations of FIGS. 10a to 10d illustrate that anumber of openings can be formed in a number of layers of the sensor 10,the layers being arranged one over the other in such a way that theopenings are also formed one over the other, so that passages 17, 17′and 17″ can be formed.

The sensors 10 shown comprise a substrate 11, a second electrode layer13 arranged thereupon, a first electrode layer 12 and also a firstinsulation layer 14, which is arranged between the first electrode layer12 and the second electrode layer 13. A first covering layer 21 and alsoa second covering layer 42 are formed on the first electrode layer 12.The first electrode layer 13 does not have an arrangement of openingsfor the forming of passages (see FIG. 10a ).

Gaps 95 are formed within the second electrode layer 13. The firstinsulation layer 14 is arranged on the second electrode layer 13 in sucha way that the openings 16 in the first insulation layer 14 are notarranged above the gaps 95.

On the other hand, the first electrode layer 12 is arranged in such away that the openings 15 in the first electrode layer 12 are arrangedabove the openings 16 in the first insulation layer 14. With the aid ofthe openings 15 in the first electrode layer 12 and the openings 16 inthe first insulation layer 14, passages 17 are formed, the side 31 ofthe first electrode layer 13 serving as the bottom 28 of the passages,in particular of blind holes and/or elongate depressions 17′, 17″.

In FIG. 10 b, the arrangement of the openings 15 and 16 in relation toone another is shown in an enlarged representation. It can be seen thata first portion 45 and a second portion 46 with openings 15 and 16 arerespectively formed both in the first insulation layer 14 and in thefirst electrode layer 12. The openings 15 and 16 arranged one over theother form in each case blind-hole-like passages 17.

Also in FIG. 10 c, a first portion 45 and a second portion 46 arerespectively formed in the first insulation layer 14 and also in thefirst electrode layer 12. Elongate openings 15, 16 are respectivelyformed in the portions 45 and 46, the elongate openings 15 and 16 beingoriented in the same directions.

According to the representation of FIG. 10d it is possible that theelongate openings 15 and 16 can also be aligned perpendicularly inrelation to the orientations shown in FIG. 10 c.

It is pointed out that some of the sensors 10 shown (FIGS. 1a -1 c, FIG.4, FIG. 5, FIGS. 8a-b and FIG. 9) are in each case only shown as adetail. The measurement of the particles preferably takes place only inthe passages 17, 17′, 17″ and not on side edges/side faces of the sensorand not on side faces/side edges of the sensor layers.

It is also possible that, in a further embodiment of the invention, allof the sensors 10 shown do not have an upper insulation layer/coveringlayer 21 and/or do not have a filter layer 27. If sensors 10 do not havean upper insulation layer/covering layer 21 and/or do not have a filterlayer 27, large particles have no influence on the signal or on themeasurement result.

With regard to a possible production process in connection with thesensors 10 according to the invention of FIGS. 1a -c, 2, 4, 5, 8 a-b, 9and FIGS. 10a -d, reference is made to the production possibilitiesalready described, in particular to etching processes and/or lasermachining processes.

At this stage it should be pointed out that all of the elements andcomponents described above in connection with the embodiments accordingto FIGS. 1a to 10d are essential to the invention on their own or in anycombination, in particular the details that are shown in the drawings.

LIST OF DESIGNATIONS

-   10 Sensor-   11 Substrate-   12 First electrode layer-   13 Second electrode layer-   14 First insulation layer-   15 Opening in first electrode layer-   16 Opening in first insulation layer-   17 Passage-   17′, 17″ Elongate depression-   18 Bonding agent layer-   19 Side face of second electrode layer-   20 Side of the first electrode layer-   21 Covering layer-   22 Side face of first electrode layer-   23 Side face of insulation layer-   24 Upper portion of covering layer-   25 Side portion of covering layer-   26 Side of covering layer-   27 Porous filter layer-   28 Bottom-   29 Opening in covering layer-   30, 30′ Particle-   31 Side of second electrode layer-   32 Peripheral region of first electrode layer-   33 Electrical contacting area of first electrode layer-   34 Electrical contacting area of second electrode layer-   35 Additional electrical contacting area of second electrode layer-   36 Strip conductor loop-   37 Side of insulation layer-   40 Pore in first electrode layer-   41 Pore in insulation layer-   42 Second covering layer-   45 First portion-   46 Second portion-   47 Central portion-   48 Frame-like portion-   50 Third electrode layer-   51 Fourth electrode layer-   60 Second insulation layer-   61 Third insulation layer-   70 Opening in third electrode layer-   71 Opening in second insulation layer-   72 Opening in fourth electrode layer-   73 Opening in third insulation layer-   80, 80′, 80″ Opening-   90 Undercut-   95 Gap-   a Direction of flow-   b Width of sensor layer-   l Length of sensor layer-   B1 Width of passage-   B2 Width of passage-   d Thickness of insulation layer-   x Longitudinal axis of the elongate depressions-   α Angle between the normal to the electrode plane and the direction    of flow-   β Angle between the longitudinal axis and the direction of flow

1.-43. (canceled)
 44. A sensor for detecting soot particles, the sootparticles being electrically conductive or polarizable, the sensorcomprising: substrate, a first electrode layer and a second electrodelayer, the second electrode layer arranged between the substrate and thefirst electrode layer; a first insulation layer disposed between thefirst electrode layer and the second electrode layer; a first openingdisposed in the first electrode layer and a second opening disposed inthe first insulation layer; wherein the first opening and the secondopening are aligned to form a first passage to the second electrodelayer.
 45. The sensor as claimed in claim 44, further comprising asecond insulation layer and a third electrode layer, the secondinsulation layer disposed between the first electrode layer and thethird electrode layer, a third opening disposed in the third electrodelayer and a fourth opening disposed in the second insulation layer, andwherein the third opening and the fourth opening are aligned to form apassage extension to the first passage to the second electrode layer.46. The sensor as claimed in claim 44, wherein the first opening isdistal from a peripheral region of the first electrode layer and thesecond opening is distal from a peripheral region of the firstinsulation layer, and wherein the third opening is distal from aperipheral region of the third electrode layer and the fourth opening isdistal from a peripheral region of the second insulation layer.
 47. Thesensor as claimed in claim 45, wherein the first electrode layer, thesecond electrode layer, or the third electrode layer comprises a metal,a metal alloy, a high-temperature-resistant metal, ahigh-temperature-resistant alloy, a platinum metal, or an alloy of ametal of the platinum metals.
 48. The sensor as claimed in claim 45,wherein the first electrode layer comprises a first material selectedfrom the group of a metal, a metal alloy, a high-temperature-resistantmetal, a high-temperature-resistant alloy, a platinum metal, or an alloyof platinum metals, wherein the second electrode comprises a secondmaterial selected from the group of a metal, a metal alloy, ahigh-temperature-resistant metal, a high-temperature-resistant alloy, aplatinum metal, or an alloy of platinum metals, wherein the thirdelectrode comprises a third material selected from the group of a metal,a metal alloy, a high-temperature-resistant metal, ahigh-temperature-resistant alloy, a platinum metal, or an alloy ofplatinum metals, and wherein the second material has a higher etchingresistance than the first material or the third material.
 49. The sensoras claimed in claim 44, further comprising a covering layer disposed ona side of the first electrode layer, the side of the first electrodelayer facing away from the first insulation layer, the covering layercomprising ceramic, a glass, a metal oxide, or a combination thereof.50. The sensor as claimed in claim 45, further comprising a coveringlayer disposed on a side of the third electrode layer, the side of thethird electrode layer facing away from the first insulation layer, thecovering layer comprising ceramic, a glass, a metal oxide, or acombination thereof, wherein the first passage is a blind hole, whereina portion of the second electrode layer is a bottom of the blind hole,and wherein the blind hole extends through the first insulation layer,the first electrode layer, the second insulation layer, the thirdelectrode layer, or the covering layer.
 51. The sensor as claimed inclaim 50, wherein the blind hole has a square cross section with asurface area in a range of 3×3 μm²-150×150 μm², a range of 10×10μm²-100×100 μm², a range of 15×15 μm²-50×50 μm², or 20×20 μm².
 52. Thesensor as claimed in claim 44, further comprising a fifth openingdisposed in the first electrode layer and a sixth opening disposed inthe first insulation layer, wherein the fifth opening and the sixthopening are aligned to form a second passage to the second electrodelayer, wherein the first passage is a first blind hole having a firstcross-sectional area, wherein the second passage is a second blind holehaving a second cross-sectional area, and wherein the firstcross-sectional area is larger than the second cross-sectional area. 53.The sensor as claimed in claim 45, wherein the first passage, thepassage extension, or a combination of the first passage and the passageextension comprises a meandering shape or a spiral shape.
 54. The sensoras claimed in claim 53, further comprising a covering layer disposed ona side of the third electrode layer, the side of the third electrodelayer facing away from the first insulation layer, the covering layercomprising ceramic, a glass, a metal oxide, or a combination thereof,wherein the first passage is a blind hole, wherein a portion of thesecond electrode layer is a bottom of the blind hole, and wherein theblind hole extends through the first insulation layer, the firstelectrode layer, the second insulation layer, the third electrode layer,or the covering layer.
 55. The sensor as claimed in claim 45, furthercomprising a covering layer disposed on a side of the third electrodelayer, the side of the third electrode layer facing away from the firstinsulation layer, the covering layer comprising ceramic, a glass, ametal oxide, or a combination thereof, wherein the first electrode layercomprises a first electrical contact area, wherein the second electrodelayer comprises a second electrical contact area, wherein the thirdelectrode layer comprises a third electrical contact area, wherein thefirst electrical contact area is connected to the first electrode layer,the second electrical contact area is connected to the second electrodelayer, the third electrical contact area is connected to the thirdelectrode layer, wherein the second electrical contact area is notoverlayed by the first insulation layer and the first electrode layer,wherein the first electrical contact area is not overlayed by the secondinsulation layer and the third electrode layer, wherein the thirdelectrical contact area is not overlayed by a covering layer, andwherein each electrical contact area is connected to a terminal pad. 56.The sensor as claimed in claim 55, wherein the first electrode layer,the second electrode layer, or the third electrode layer comprises astrip conductor loop, strip conductor loop being a heating coil, atemperature-sensitive layer, a shielding electrode, or a combinationthereof, wherein the first electrode layer, the second electrode layer,or the third electrode layer comprising the strip conductor loopcomprises further a fourth electrical contact area not overlayed by oneof the insulation layers or an electrode layer, and wherein the fourthelectrical contact area is connected to the terminal pad.
 57. A sensorsystem comprising: the sensor of claim 45, and a controller or a controlcircuit, the controller or the control circuit for operating the sensorin a measuring mode, in a cleaning mode, in a monitoring mode, or acombination thereof.
 58. A method for controlling the sensor as claimedin claim 45, the method comprising the step of: operating the sensor ina measuring mode, in a cleaning mode, in a monitoring mode, or acombination thereof.
 59. A method of making a sensor for detecting sootparticles, the soot particles being electrically conductive orpolarizable, the sensor comprising a substrate; a first electrode layerand a second electrode layer, the second electrode layer arrangedbetween the substrate and the first electrode layer; a first insulationlayer disposed between the first electrode layer and the secondelectrode layer; a second insulation layer and a third electrode layer,a third opening disposed in the third electrode layer and a fourthopening disposed in the second insulation layer, and wherein the thirdopening and the fourth opening are aligned to form a passage extensionto the first passage to the second electrode layer, the methodcomprising the steps of: laminating the first electrode layer, thesecond electrode layer, the third electrode, the first insulation layer,and the second insulation layer to form a laminate, the first insulationlayer being disposed between the first electrode layer and the secondelectrode layer, the second insulation layer disposed between the firstelectrode layer and the third electrode layer, subsequently forming apassage through the first electrode layer, the third electrode layer,the first insulation layer, and the second insulation layer, ending thepassage to have a bottom formed by a portion of the second electrodelayer.
 60. The method as claimed in claim 59, wherein the passage isformed as a blind hole by etching, plasma-ion etching, or successiveetching adapted to each layer being etched.
 61. The method as claimed inclaim 60, wherein the passage is formed as a blind hole or as anelongate depression by etching, plasma-ion etching, or successiveetching adapted to each layer being etched, and wherein the firstinsulation layer or the second insulation layer is etching-resistantlayer, the blind hole or a portion of the elongate depression beingformed in the insulation layer by a conditioning process with phaseconversion of the first insulation layer or the second insulation layer.62. The method as claimed in claim 59, wherein the passage is formed asa blind hole, a subportion of the blind hole, an elongate depression, ora subportion of the elongate depression by irradiation, whereinirradiation is performed with electromagnetic waves, charged particles,or electrons, wherein a radiation source, a wavelength, a pulsefrequency of a radiation, or energy of the charged particles beingadapted individually to each layer being irradiated.
 63. A method ofmaking a sensor for detecting soot particles, the soot particles beingelectrically conductive or polarizable, the sensor comprising asubstrate; a first electrode layer and a second electrode layer, thesecond electrode layer arranged between the substrate and the firstelectrode layer; a first insulation layer disposed between the firstelectrode layer and the second electrode layer; a second insulationlayer and a third electrode layer, a third opening disposed in the thirdelectrode layer and a fourth opening disposed in the second insulationlayer, and wherein the third opening and the fourth opening are alignedto form a passage extension to the first passage to the second electrodelayer, the method comprising the steps of: laminating the firstelectrode layer, the second electrode layer, the third electrode, thefirst insulation layer, and the second insulation layer to form alaminate, the first insulation layer being disposed between the firstelectrode layer and the second electrode layer, the second insulationlayer disposed between the first electrode layer and the third electrodelayer, wherein the first electrode layer, the second electrode layer,the third electrode, the first insulation layer, or the secondinsulation layer are structured by a lift-off process, an ink-jetprocess, a stamping process one over the other forming a passage to thesecond electrode layer.
 64. A method of using the sensor of claim 45,the method comprising the step of: directing a flow (a) of the sootparticles to not impinge perpendicularly on a plane (x, y) of the thirdelectrode.
 65. A method of using the sensor of claim 56, the methodcomprising the step of: detecting electrically conductive or polarizableparticles, and adjusting an angle α between a normal (z) to a plane (x,y) of the first electrode layer and a direction of a flow (a) of theparticles is 1 degree or more, 10 degrees or more, or 30 degrees ormore.